1. Field of the Invention
This invention relates in general to systems for chemical vapor deposition of materials on substrates, and more particularly to a chemical vapor deposition system having improved substrate loading, off-loading, and handling sub-systems which interact with at least one especially configured processing subsystem having a reaction chamber, susceptor and heating sub-assemblies for precision control of the deposition process.
2. Discussion of the Related Art
In the electronics art, it has long been a practice to employ chemical vapor deposition techniques for depositing various materials on substrates or wafers, as part of the process for manufacturing semiconductor devices. Chemical vapor deposition processes includes passing of a reactant gas, which contains the material to be deposited, over the substrates for forming, or growing a compound on the substrates as a result of thermal reaction or decomposition of the various gaseous materials.
The equipment used to accomplish such a process is of various configurations but will include the basic components of a reaction chamber, a heating system and a gas flow system. Of course these various components are configured in accordance with the tasks to be accomplished. For example, when the number of substrates to be processed is small, the reaction chamber may be in the form of a bell jar, but in high quantity production work considerably more sophisticated equipment is needed.
For some time now, batch processing equipment has been used for accomplishing the chemical vapor deposition processes in production environments and batch processing equipment may be categorized as being of two basic types, namely horizontal gas flow systems and vertical gas flow systems. A horizontal gas flow system generally includes a platform, or susceptor as it is referred to in the art, which is located in a horizontally disposed reaction chamber with the reactant gas flowing in a horizontal path across the susceptor. In a vertical gas flow system, a horizontally disposed susceptor, or an upstanding multi-surface barrier shaped susceptor, is located in a vertically disposed reaction chamber with the reactant gas being caused to flow in a substantially vertical path and around the susceptor. In either case, these susceptors are configured to support a multiplicity of relatively small substrates, i.e. in the neighborhood of 2 to 5 inches in diameter, for simultaneously depositing, materials on the multiplicity of substrates. While simultaneous deposition of materials on a multiplicity of substrates is desirable from a manufacturing standpoint, it has some drawbacks from a quality standpoint.
The first problem associated with batch, or multi-substrate processing relates to the reactant gas, which contains the atoms of the deposition materials. As the gas flows over the surfaces of the substrates and the susceptor, deposition of the materials results in changes in the concentration of the deposition materials in the reactant gas. Consequently, as the reactant gas flows across or over the length of these relatively large susceptors, across each individual substrate and across a multiplicity of such substrates, different rates of growth of the deposited layer of material have been found. A second problem is that of temperature control which is critical at the elevated temperatures needed for proper deposition. It is difficult, if not impossible, to control the temperature within the critical tolerances at all the desired locations within the relatively large reaction chambers. This results in different deposition layer thickness from one substrate to another, and can even produce varying thickness within the individual substrates. Still another problem is contamination which can result from various factors such as the handling techniques used to load and unload the substrates, the introduction of the reactant gas into the reaction chamber, and the like.
These problems and drawbacks, as well as other factors, all contribute to significant problems as the semiconductor devices and the uses to which they are put become more sophisticated. As a result, many changes and improvements have been made in the equipment that is used to simultaneously process a multiplicity of substrates. For example, some equipment manufacturers are now using automated loading and off-loading devices, as opposed to hand-loading techniques, to eliminate, or at least substantially reduce contamination resulting from human handling. Further, the second type of susceptor discussed above, i.e. the upstanding barrel shaped structure, is being rotated in some instances about its vertical axis to rotate the multiplicity of substrates about that same axis within the reaction chamber. Such barrel rotation is being done for averaging purposes, that is, temperature averaging and reactant gas flow averaging. Obviously these and other things which are being done to improve the simultaneous multi-substrate processing techniques have helped. However, there are practical limits which many feel will ultimately make the batch processing techniques unacceptable or at least undesirable. One of the limitations is that of the equipment not being very well suited for handling larger diameter substrates. The economics of larger diameter substrates are causing many manufactures of semiconductor devices to use larger substrates. However, increasing the size of the substrate is causing some problems with regard to temperature differentials across the substrate, decreasing concentrations of the deposition material as it is carried across the substrate, and the like.
Therefore, steps are being taken now by some equipment manufacturers to make suitable single substrate processing equipment which is significantly simpler in so far as controlling the various factors involved in chemical vapor deposition. Single substrate chemical vapor deposition equipment becomes inherently more desirable than multi-substrate equipment as the manufacturers of semiconductor devices change to larger substrates, i.e. 6 to 8 inches in diameter or even larger. One important consideration is the cost at risk when processing one substrate as opposed to the simultaneous multi-substrate processing. That is, if something goes wrong, the monetary loss is far less with one substrate than it is with a plurality of substrates.
Various prior art components and sub-systems have been devised for use in building single substrate processing chemical vapor deposition systems. For example, loading and unloading of substrates into such systems may be handled in various ways with the most pertinent prior art structure being a cassette elevator available from the Brooks Automation Co., a division of Aeronca Electronics, Inc., One Executive Park Drive, North Billerica, Miss. 01862. The cassette elevator, which is identified as Product No. 6200, includes a vacuum chamber for receiving a plurality of substrates that are carried in a cassette with the cassette being supported on a platform. The platform is vertically movable by means of an elevating mechanism which brings the substrates one at a time into alignment with an access port. An isolation valve such as that available as Product No. 3003 from the above identified Brooks Automation Co., is located at the access port of the elevating mechanism for closing the vacuum chamber except during extraction of the individual substrates. Both the elevating mechanism and the isolation valve provide a controllable environment for receiving and loading the substrates into a processing system.
The Brooks Automation Co. also markets a vacuum transport station under the name Vacu-Tran.TM. for extracting the substrates one at a time from the elevating mechanism described above. The transport station includes a housing which is coupled to the isolation valve described above, and a robot arm structure is located in the housing. The robot arm structure includes a rotatable plate having an extensible and retractable arm arrangement thereon, with a pallet or spatula on the distal end of the arms. With the plate and arms rotated so as to align with the access port of the elevating mechanism and the isolation valve open, the arms are extended to move the pallet into position below a substrate, and then the entire arm structure is raised to lift the substrate so that it is carried on the pallet out of the cassette. The arms are then retracted to extract the substrate from the elevating mechanism, and then the arm assembly is rotated to another position and extended once again so as to pass through another isolation valve into a suitable reaction chamber. This particular handling system relies on the weight of the substrate to hold it in place on the pallet and another prior art structure includes a similar arm arrangement which further includes a vacuum outlet on the pallet for a more positive attachment to the underside of the substrates.
The operation of the above described prior art loading system can be reversed for extracting a processed substrate from the reaction chamber and returning it to the same cassette from which it was extracted, or alternatively, to another cassette provided in a second elevating mechanism provided solely for off-loading of processed substrates.
While the above described loading, handling and off-loading structures are significantly better than hand operations, and other prior art loading and handling mechanism which are not relevant to the present invention, they are less than completely satisfactory. One of the prime considerations in modern chemical vapor deposition systems is to hold contamination to an absolute minimum, and prevent it entirely, if possible. In that the vacuum chamber of the elevating mechanism must be opened from time to time for insertion and extraction of cassettes, environmental contamination will enter the vacuum chamber. The isolation valve located at the access port of the elevating mechanism is needed to prevent contaminants from passing through the vacuum chamber of the elevating mechanism into the housing of the transport system during the time when the vacuum chamber is open to the environment, and such an isolation valve is expensive. However, the main problem with this prior art system is in the robot arm structure which slides under and carries the substrates on the pallet from the elevating mechanism to the reaction chamber and back again when processing is completed. First of all, such a substrate handling technique cannot possibly place a substrate on a flat continuous surface, such as an ideally configured susceptor, which is used in the reaction chamber, due to the pallet of the robot arms being in supporting engagement with the bottom surface of the substrate. Therefore, some sort of less than ideal susceptor configuration must be provided in the reaction chamber if is it is to be used with the prior art robot arm handling mechanisms. Secondly, damage often results from the pallet coming into mechanical contact with the substrate. Also, contaminants in the form of airborne particles can settle on the top surface of the substrate and this reduces the yield of the substrates and destroys circuit integrity.
As was the case with the above discussed batch processing chemical vapor deposition systems, the reaction chambers used in single substrate processing systems may be categorized as either a horizontal gas flow system or a vertical gas flow system. However, the susceptors being used in the single substrate reaction chambers consist essentially of a planar platform or base upon which the substrate rests during the deposition process, and those susceptors contribute nothing further to the deposition process with regard to improving the problems of depletion of the material carried by the reactant gas as it flows past and around the substrate, and with regard to improved temperature sensing and control.
Therefore, a need exists for a new and improved single substrate chemical vapor deposition system which enhances the process and thereby helps in eliminating, or at least reducing, the problems and shortcomings of the prior art systems.